The present invention relates to optical components used in fabricating optical devices. More specifically, the present invention relates to an optical module which carries an optical, optical-electrical or optical mechanic component.
Optical devices are being increasingly used in various industries and technologies in order to provide high speed data transfer such as in fiber optic communication equipment. In many applications there is a transition or an incorporation of optical devices where previously only electrical devices were employed. An optical device typically consists of a number of components which must be precisely assembled and aligned for the device to operate and function efficiently. Example components include fibers, waveguides, lasers, modulators, detectors, gratings, optical amplifiers, lenses, mirrors, prisms, windows, etc.
Historically, optical devices such as those used in fiber optic telecommunications, data storage and retrieval, optical inspection, etc. have had little commonality in packaging and assembly methods. This limits the applicability of automation equipment for automating the manufacture of these devices since there is such a disparity in the device designs. To affect high volume automated manufacturing of such devices, parts of each individual manufacturing line have to be custom-designed.
In contrast, industries such as printed circuit board manufacturing and semiconductor manufacturing have both evolved to have common design rules and packaging methods. This allows the same piece of automation equipment to be applied to a multitude of designs. Using printed circuits as an example, diverse applications ranging from computer motherboards to cellular telephones may be designed from relatively the same set of fundamental building blocks. These building blocks include printed circuit boards, integrated circuit chips, discrete capacitors, and so forth. Furthermore, the same automation equipment, such as a pick and place machine, is adaptable to the assembly of each of these designs because they use common components and design rules.
Further complications arise in automated assembly of optical devices. Such assembly is complicated because of the precise mechanical alignment requirements of optical components. This adds to problems which arise due to design variations. The problem arises from the fact that many characteristics of optical components cannot be economically controlled to exacting tolerances. Examples of these properties include the fiber core concentricity with respect to the cladding, the location of the optical axis of a lens with respect to its outside mechanical dimensions, the back focal position of a lens, the spectral characteristics of a thin-film interference filter, etc. Even if the mechanical mounting of each optical element were such that each element was located in its exact theoretical design position, due to the tolerances listed above, the performance specifications of the optical device may not be met.
To appreciate the exacting alignment requirements of high performance optical devices, consider the simple example of aligning two single mode optical fibers. In this example, the following mechanical alignments are required to ensure adequate light coupling from one fiber to the other: the angle of the fibers with respect to each other, the fiber face angle, the transverse alignment (perpendicular to the light propagation direction) and the longitudinal spacing (parallel to the light propagation direction).
Typical single mode optical fibers used in telecommunications for the 1.3 μm to 1.6 μm wavelength band have an effective core diameter of about 9 microns and an outside cladding dimension of 125 microns. The typical tolerance for the concentricity of the core to the outside diameter of the cladding is 1 micron. If the outside claddings of the two fibers were perfectly aligned and there is no angular misalignment or longitudinal spacing, the cores may still be transversely misaligned by as much as 2 microns. This misalignment would give a theoretical coupling loss of about 14 percent or 0.65 dB. This loss is unacceptable in many applications. Techniques using active alignment, such as that shown in U.S. Pat. No. 5,745,624, entitled “AUTOMATIC ALIGNMENT AND LOCKING METHOD AND APPARATUS FOR FIBER OPTICAL MODULE MANUFACTURING”, issued Apr. 28, 1998 to Chan et al., can be employed to improve coupling efficiency.